A heat exchanger is already known from Japanese Patent Application Laid-open No. 61-153500, which includes a large number of projections which are formed on heat-transfer plates defining high-temperature fluid passages and low-temperature fluid passages, and which are coupled together at tip ends of the projections.
In a heat exchanger including first and second heat-transfer plates disposed radiately to define the high-temperature fluid passages and the low-temperature fluid passages alternately in a circumferential direction, the sectional area of a flow path in each of the high-temperature fluid passages and the low-temperature fluid passages is narrower on its radially inner side and wider on a radially outer side, and the level of the projections formed on the heat-transfer plate is lower on the radially inner side and higher on the radially outer side. As a result, there is a possibility that the heat transmission coefficient of the heat-transfer plate and the mass flow rate of the fluid may be non-uniform radially, whereby the total heat-exchange efficiency is reduced, and an undesirable thermal stress is produced.
There is also a conventionally known heat exchanger which is described in Japanese Patent Application Laid-open No. 58-223401, which includes a plurality of heat-transfer plates disposed at a predetermined distance, and high-temperature fluid passages and low-temperature fluid passages defined between adjacent heat-transfer plates by bonding tip ends of bank-shaped projection stripes formed on the heat-transfer plates to each other.
When the tip ends of the projection stripes formed at end edges of the adjacent heat-transfer plates are bonded to each other by brazing, the end edges of the heat-transfer plates may be curved in a direction opposite from a direction of protrusion of the projection stripes due to a thermal influence of the brazing, whereby the sectional area of a flow path in each of an inlet and an outlet of the fluid passage defined between the adjacent heat-transfer plates may be reduced in some cases. Moreover, if the projection stripes are disposed on folding lines for folding the first and second heat-transfer plates in a zigzag fashion, the rigidity of those portions of the first and second heat-transfer plates which correspond to the projection stripes is increased, whereby it is difficult to carry out the folding operation. Moreover, the shape of a folded area at each of the folding lines may be destroyed at such portions to produce a gap between the projection stripes, whereby the fluid may be leaked from such gap in some cases, resulting in a reduction in a heat transfer efficiency.